Catalysts for decomposition of exhaust gas constituents have been studied and used previously. The catalysts of the present invention would be useful in automobile catalytic converters, and would be required and expected to reach corresponding regulatory requirements. These materials, involving simultaneous formation of the catalytically active component and the surrounding metal oxide lattice, will offer improved performance at lower costs for exhaust pollutant removal, as compared to mixtures of their components as found in currently used catalysts.
Catalysts currently used in converters for exhaust gas pollutant removal utilize combinations of platinum and rhodium supported on aluminum oxide. Such catalysts can remove over 90% of evolved exhaust gas pollutants. However, the cost of such catalysts is an undesirable feature: a monolith (a supporting ceramic or metal structure) for a typical catalytic converter may contain $30-40 worth of precious metals. In contrast, the catalysts of the present invention may possess efficiencies, usually expressed in terms of "cost-based activities" (pollutant removal rate divided by total cost of active catalyst metal) of between 1 and 100 times those of currently used catalysts.
Several requirements exist for pollutant mitigation catalysts. The first is that they be capable of adsorbing the reactant (pollutant) to be removed while simultaneously weakening the bonds between the constituents of the pollutant molecule. This may involve several chemical features. For example, the ability to form bonds to hydrogen (from decomposition of hydrocarbons), carbon monoxide, and nitrogen oxides are all requirements of the catalyst, since formation of these bonds results in rupture of bonds in pollutant molecules and easier decomposition (of nitrogen oxides or hydrocarbons) or oxidation of the molecules (carbon monoxide or hydrocarbons).
The second requirement is that reactions between adsorbed hydrocarbon and carbon monoxide reactants and adsorbed oxygen occur. This combustion process permits the pollutant species to be readily removed from the catalyst surface to provide for a renewal of available catalyst sites. This requirement may be augmented by catalyst constituents that permit adsorbed oxygen atoms to move more freely to and from the reaction site.
The third requirement is that the catalyst display long lifetime towards exposure to exhaust gas constituents. Specifically, this requirement will be effected by the incorporation of catalyst constituents imparting basicity and oxygen mobility to the catalyst. The basicity enhances the removal of hydrogen from hydrocarbons and promotes incorporation of oxygen into the hydrocarbon fragments while preventing carbon deposition. The oxygen mobility prevents the blockage of catalyst sites by adsorbed oxygen. The active catalyst site must also allow for adsorbed pollutant fragments to have sufficient mobility over the catalyst surface to allow the reaction to proceed. For example, oxygen must possess sufficient mobility to interact chemically with adsorbed CO or hydrocarbons.
The fourth requirement is that a component be present that allows for excess oxygen to be taken up from the active site during periods of the engine cycle in which oxygen is present in excess in the exhaust gases, followed by its release during periods of the engine cycle in which oxygen is at relatively low levels, as occurs in engine exhausts due to varying levels of reducing agents (carbon monoxide or hydrocarbons) in the exhaust stream.
The fifth requirement is that the catalyst possess thermal and chemical stability under normal operating conditions.
The sixth requirement of the catalyst is that its rendering into operating form be simple, rapid, and inexpensive.
The above chemical requirements can be met by use of transition metals (Groups IIIB-IB) for the catalytically active component, in conjunction with lanthanides or actinides, and elements from the right hand side of the Periodic Table of the Elements (post transition elements from Groups IIIA-VIA). A catalyst capable of achieving mitigation of hydrocarbons, carbon monoxide, and nitrogen oxide would contain a platinum group (i.e. Pt, Pd, Ir, Rh, Os, or Ru) metal or first row transition metals (Cu, Co, Mn, Cr, or V) and lanthanum group oxides such as lanthanum oxide and cerium oxide. Of particular relevance to the present invention are palladium and copper catalysts utilizing lanthanum oxide and cerium oxide. Simple mixtures or sequential deposits of individual component oxides or metals do not allow for the possibility of the enhancement of redox (oxidation-reduction) behavior, stability, and oxygen or hydrogen transport that is attainable in particular doped binary or ternary materials disclosed in the present invention.
Previously used approaches employed sequential deposition of a washcoat of rare earth elements onto ceramic supports by dipping in a solution or slurry of salts of these species or by spraying a solution of these salts onto a monolith, followed by firing of the washcoat and subsequent deposition and reduction of a solution of a complex of the precious metal (Pt, Rh, Pd, or Ir). These approaches obviously lead to higher catalytic converter manufacturing costs when compared with the present approach. Additionally, there are necessarily separate phases of catalyst and support (alumina) as well as separate phases arising from washcoats, potentially leading to lower catalytic performance than is expected from the catalysts of the present invention.
In the present approach, a single-phase oxygen-deficient metal oxide material containing the catalytically active component is formed from a mixture of the constituent metal ions. This approach has the advantages of lower manufacturing costs as well as the enhanced catalysis resulting from the formation of single-crystallographic-phase catalysts.
In U.S. Pat. No. 5,234,881 to Narula et al, lanthanum and palladium are combined in the stoichiometry La.sub.2 Pd.sub.2 O.sub.5 or La.sub.4 PdO.sub.7 and supported on a substrate. However, no provision is made for the fourth requirement above: that is, a component such as Ce.sup.4+ is not available. Additionally, the amount of palladium must, by the nature of that invention, be present in a stoichiometric ratio relative to the lanthanum component. This is in contrast to the present invention in which the transition metal (catalytically active component) is not necessarily present as a major stoichiometric component and, in fact, may be present to any fraction of the total weight of the catalyst, resulting in lower overall cost when the precious metal (e.g. Pd) fraction is reduced. Additionally, the present invention may incorporate less expensive catalysts than Pd, such as Cu, Ni, or other transition metals.
Also related to the subject matter of the present invention in respect to components used in catalyst preparation are U.S. Pat. Nos. 4,791,091; 4,868,149; 4,919,902; and 4,960,574 which describe a cerium oxide, lanthanum oxide, palladium oxide, and rhodium metal catalyst dispersed on an alumina coating on a honeycomb carrier. It was disclosed in U.S. Pat. Nos. 4,868,149 and 4,960,574, both to Bricker, that lanthanum replaces at least 3% of the cerium in the cerium oxide lattice. Additionally, U.S. Pat. Nos. 4,791,091 and 4,919,902, both to Bricker, disclosed an approach where the lanthanum oxide component was dispersed onto an alumina powder by suspending the alumina in a solution of a salt of lanthanum, followed by calcination. However, the catalyst preparation of those patents involves sequential deposition of components on a supporting structure, whereas in the present invention the carrier and active site are simultaneously formed and a characteristic crystallographic structure is obtained through the preparation, thus gaining an advantage in meeting the sixth requirement above. The formation of a crystallographically well-defined oxygen deficient phase as in the present invention allows for enhanced oxygen ion transport and oxygen uptake by oxidizable components, leading to superior characteristics in the third through the sixth requirements above.
Binary oxide catalysts possessing the general formulas A.sub.x M.sub.y O.sub.z (where A is an alkali metal, alkaline earth, rare earth, first row transition metal, or Y or Zr; and M is a metal from the group Ir, Rh, Pt, Pd, and Ru) were disclosed in U.S. Pat. No. 4,127,510 to Harrison et al. as achieving catalytic activity for three-way catalytic removal of exhaust pollutants. However, that invention is distinct from the present invention in that the metal M is only derivable from the group Ir, Rh, Pt, Pd, and Ru, and only forms compound oxides when the precious metal M is present in a stoichiometric amount, thus requiring that a very large fraction of the catalyst (&gt;10% by weight) is present as precious metal, in distinct contrast to the present invention where weight fractions of less than 2% of precious metals are expected to be required.
Another perovskite-based catalyst is that documented in U.S. Pat. No. 5,015,616 to Sekido, et al. However, the Sekido catalyst does not possess the activity for use in catalytic removal of pollutants that is competitive with current approaches, lacking the active platinum group metals or copper as active site constituents. Perovskite materials do not intrinsically involve oxygen deficiencies, as do the materials of the present invention which are oxygen deficient by their nature. Due to its intrinsic oxygen ion vacancies, the present invention possesses enhanced oxygen ion mobility leading to greater catalytic activity.
A multifunctional catalyst that is formed by sequential or simultaneous deposition of cerium oxide, uranium oxide, rare earth oxide, and active metal (i.e Pt, Pd, Ir, Rh, Ru, Ag, or Au) is described in U.S. Pat. No. 5,108,978 to Durand, et al.
This approach differs from the present invention in that a single distinct catalyst crystallographic phase was not prepared, resulting in requirements 2-6 above not being met to the same extent as with the present invention.
Other modifications of the platinum group metal/redox resistant rare earth oxide/oxidizable-reducible rare earth oxide catalysts are documented in U.S. Pat. Nos. 4,587,231; 4,760,044; 4,923,842; 5,041,407; 5,116,800; and 5,248,650. None of these inventions involve the intentional formation of structurally well-defined catalyst compounds (catalyst properties depend on catalyst structure).
Literature related to ceria-based washcoat catalysts include articles concerning Pd--La.sub.2 O.sub.3 /Al.sub.2 O.sub.3 (H. Muraki, H. Shinjoh, H. Sobukawa, K. Yokota, and Y. Fujitani, Ind. Eng. Chem. Prod. Res. Dev., 25, 202(1986)), Al.sub.2 O.sub.3 and CeO.sub.2 /Al.sub.2 O.sub.3 supported palladium and palladium-platinum catalysts (K. M Adams and H. S. Gandhi, SAE Paper No. 930084), WO.sub.3 modified Pd supported on Al.sub.2 O.sub.3 (Ind. Eng. Chem. Prod. Res. Dev., 22, 207(1983)), and CuO and Cr.sub.2 O.sub.3 utilizing CeO.sub.2 washcoats. The latter (CuO and Cr.sub.2 O.sub.3) catalysts demonstrated poor activity toward nitrogen oxide decomposition and were generally intolerant towards sulfur in the exhaust stream: copper and chromium-based catalysts are well-known to have much less catalytic activity as compared to platinum-based catalysts. The poor activity problem is anticipated to be absent with the present invention because of the possibility of incorporating the aforementioned catalyst components or more active catalytic components (e.g. Pd, Pt, Ir, or Rh) into the single phase materials of the present invention possessing attributes such as superior oxygen mobility anticipated to enhance catalytic activity. The sulfur intolerance problem may be minimized by use of active catalyst sites forming relatively weak bonds with sulfur such as the platinum group metals Pt, Pd, Ir, and Rh.